Dendritic cell-based immune therapies that exploit natural mechanisms of antigen presentation represent the most promising non-toxic method of cancer treatment. It may be used as a sole treatment, or as an adjuvant for other types of therapies such as e.g. surgery, irradiation and chemotherapy. The strategy is based on ex vivo manipulation and reintroduction of cellular products to circumvent immune competences for the purpose of inducing tumor specific immune responses. Thus, the ultimate goal of such dendritic cell-based immune therapies is the induction of tumor-specific effector cells in vivo and recent advances has focused on CD8+ cytotoxic T lymphocytes (CTL) capable of recognizing and killing tumor cells. In addition, the treatment of infectious diseases such as e.g. HIV may benefit from dendritic cell-based vaccination strategies.
Antigen Presentation
Induction of tumor specific immune responses require the engagement of professional antigen presenting cells (APC) expressing Major Histocopatibility Complex (MHC) molecules as well as membrane bound and secreted co-stimulatory molecules. Furthermore, such APC must be able to take up, process and present antigens in association with MHC molecules.
Dendritic cells (DC) are the professional APC of the immune system with the ability to activate both naïve and memory T cells. The stages leading to DC maturation are associated with certain properties of the cell. Immature DC are particularly good in taking up extra-cellular antigens by phagocytosis or pinocytosis and processing the antigens to peptides in the endocytotic compartment such as endosomes and phagosomes. Here the peptides are bound to MHC class II molecules. Immature DC do also have the unique ability of loading the peptides from exogenous proteins to the MHC class I pathway of presentation, a process called cross-presentation.
The ability to efficiently stimulate an immune response by activating CD4+ type I helper T-cells (Th1 cells) and CD8+ cytotoxic T cells (CTL) is crucially dependent on a mature DC. Only fully mature DC equipped with a panel of membrane bound co-stimulatory and accessory molecules such as e.g. CD40, CD80, CD83, CD86 and MHC class II may efficiently induce proliferation and differentiation of antigen-specific T lymphocytes1.
A significant role of the co-stimulatory activity of DC is provided by secreted cytokines in particular IL-12p70. Its role in the activation of T cells and their polarization to a Th1 type response was clearly demonstrated by Heufler et al. (1996)1. Furthermore, a good correlation between the presence of IL-12-expressing mature DC in the tumor and the survival of the patient was reported by Inoue et al. (2005). Mature DC for vaccination purpose should produce limited amounts of the Th1 cell inhibitory cytokine IL-10.
CCR7 is the receptor for the chemokines CCL19 and CCL21 which are produced by stroma cells in lymph nodes. DC expressing sufficient levels of activated CCR7 migrate to the lymph node in response to CCL19 or CCL212. Here they meet T lymphocytes and may initiate an immune response.
Protocols for Generation of Mature DC
Many protocols for the generation of mature DC have been described. The currently most often used “standard” protocol for induction of DC employs a maturation cocktail consisting of IL-1beta, IL-6, TNF-alpha and prostaglandin E2. In spite of migratory activity due to CCR7 and immuno stimulatory activity in vivo, DC matured by this cocktail generates DC with reduced ability to produce IL12p703 
A second group of DC maturation protocols comprises polyinosinic:polycytidylic acid, poly-(I:C). It is usually used in combination with cytokines such as TNF alpha, IL-1 beta, IFN-gamma and IFN-alpha. DC generated by this method produces IL-12p70, but they usually express low levels of CCR7. Low levels of CCR7 expression characterized for DC obtained in the presence of poly-(I:C) restrict their in vivo migration to lymph nodes.
Recently, a published patent application US2005/0003533A1 disclosed a method for maturation of dendritic cells expressing CCR7 which subsequently upon CD40L stimulation could be induced to produce IL-12p70.
There is therefore still an unmet requirement for development of standardized methods for generating mature dendritic cells expressing high levels of activated CCR7 and which also produce sufficient amount of IL-12p70.
Furthermore, despite the efforts of many investigators, dendritic cell-based vaccines for use in cancer therapy have in general provided immune responses with modest clinical efficacy. These vaccines have mainly been produced by ex vivo manipulation and antigen-loading of autologous DC. Increasing demands with respect to patient safety requires high level of reproducibility and compliance with regulatory issues. Thus, there is a strong need for methods that generate properly equipped DC which efficiently induce immune responses and in particular provide improved clinical responses.
In addition, ex vivo generated DC could also be implemented as therapeutic vaccine in treatment of some chronic infectious diseases such as HIV and hepatitis B and C, where traditional vaccine approach is not working efficiently. The results of the preclinical and first clinical4-5 studies indicate that DC-based immunotherapy could be a promising strategy for treatment of patients with chronic infections by HIV-1 and hepatitis B and C. As with cancer immunotherapy, efficient clinical response against these intracellular infectious agents is associated with induction of Th1 helper response required for development of CD8+ effector cells5. Therefore, one can expect that ex vivo generated dendritic cells should have the same characteristics both for treating cancer and chronic infectious diseases.